Bench-Scale Development of Microbial Chain Elongation as a Bioremediation Technology for Chlorinated Ethenes

Chlorinated ethenes are among the most prevalent legacy contaminants affecting groundwater quality. A common treatment for chlorinated ethenes in the subsurface is in situ anaerobic bioremediation where the organohalide-respiring bacteria, Dehalococcoides mccartyi, convert the contaminants to non-toxic ethene via hydrogen (H2) dependent reductive dehalogenation. Typically, D. mccartyi obtain H2 through the fermentation of organic substrates by fermentative bacteria. However, stimulation of H2 competing processes causing production of methane (a potent greenhouse gas), rapid substrate consumption of simple substrates, and well/pore clogging by viscous complex substrates often challenge bioremediation, leading to slow rates of dehalogenation or stalls at chlorinated intermediates.This dissertation details the potential of microbial chain elongation as a technology for bioremediation of chlorinated ethenes. In chain elongation, bacteria reliably produce H2 and carboxylates (e.g., butyrate (C4)) using simple compounds (e.g., ethanol (C2) and acetate (C2)) as substrates. Under certain conditions, production of alcohols (e.g., butanol (C4)) can also occur. Here, chain elongation was demonstrated to drive reductive dehalogenation of trichloroethene via direct rapid-release H2 and slow-release H2 during fermentation of elongated products. Results showed chain elongation suppressed methanogenesis, supporting chain elongation as a potential solution for bioremediation when typical fermentable substrates do not meet treatment goals. Next, the potential for chain elongation was evaluated using groundwater and soil from a Superfund site experiencing challenges with bioremediation. Soils from the site were found to contain chain elongating bacteria, while groundwater not previously stimulated with ethanol and acetate was steered to chain elongate with bioaugmentation. Additional chain elongation substrate combinations relevant to bioremediation were identified. Results are being used to inform the design of a pilot study at the site. Lastly, this research identified and demonstrated higher ethanol concentrations, higher total pressures, and higher H2 partial pressure improves chain elongation activity and production of butanol, an important biofuel. These results aid in efforts to make chain elongation relevant as a bioprocess in a circular economy and bioremediation. Cumulatively, this dissertation research demonstrated the potential of chain elongation in bioremediation of chlorinated ethenes, indicating it should be considered when evaluating solutions for contaminated sites.